Powered nailers and staplers are well-known in the art and have come into wide-spread use. This is true because they can drive fasteners more rapidly and more precisely than can be accomplished manually, In their most common form, such powered nailers and staplers are actuated by compressed air, necessitating the presence of an air compressor and long lengths of hose.
More recently, there has been interest in electrically powered nailers and staplers, requiring only a source of electrical energy at the use site. Electrical energy is always present at a construction site. Such tools are also appropriate for the home market where electrical energy is readily available.
Prior art workers have devised many types of electromechanical fastener driving tools. For example, U.S. Pat. Nos. 4,042,036; 4,204,622; and 4,323,127 each teaches an electric impact tool wherein the driver is frictionally moved through a working stroke by means of two counterrotating flywheels, each flywheel being provided with its own electric motor. U.S. Pat. No. 4,121,745 also teaches an electric impact tool utilizing counterrotating flywheels to frictionally move the driver through its working stroke. In this reference, however, one flywheel is directly driven by an electric motor, while the other flywheel is driven by the same electric motor by means of pulleys and an elastomeric belt, gear means, or the like.
U.S. Pat. Nos. 4,189,080 and 4,298,072 teach electromechanical fastener driving tools wherein the driver is moved through a working stroke by means of a single rotating high-speed flywheel. The driver is engaged between the single flywheel and a support element. The preferred form of support element comprises a low inertia roller. Other support means, such as a linear bearing or a Teflon block, could be used to accomplish the same purpose, as is taught in these references.
Electromechanical tools of the general class just described can be used to drive nails, staples or other fastening means. For purposes of an exemplary showing, the present invention will be described in terms of its application to an electromechanical nailer. It will be understood by one skilled in the art, however, that the teachings of the present invention are equally applicable to electromechanical staple driving tools.
All electromechanical fastener driving tools of the type to which the present invention is directed share a common problem. This problem is one of build-up of foreign material on the driver and transfer of the foreign material from the driver to the flywheel or flywheels. Ultimately, a good drive is no longer possible because friction between the driver and the one or more flywheels is lost.
For example, it is common practice to arrange nails in the tool magazine in parallel spaced relationship and to maintain them in this relationship through the use of strips of tape coated with a thermal plastic hotmelt glue. It is also common practice to coat at least the initial driven portion of each nail shank with a resin based coating, or the like, to assist the nail's penetration of the workpiece and to increase the nail's holding power, once driven.
Since the driver is moving through its working stroke by means of frictional engagement with at least one flywheel, the driver will tend to get hot during use of the tool. In fact, the driver gets hot enough to melt the hotmelt glue or the coating on the nail, or both. As the driver moves between the flywheels (or the flywheel and a back-up means) under a squeeze force, the melted material builds up in front of the driver-flywheel contact area until a planing or floating action occurs, and the driver-flywheel contact is actually reduced enough to lose friction and thus the driving force.
A driver for a tool of the type contemplated by the present invention generally comprises an elongated blade-like element having parallel, planar, relatively wide forward and rearward working faces with narrow edges. The driver may be provided with a lead-in taper or ramp at the beginning of one or both working surfaces. The working faces are engaged by the counterrotating flywheels, or by one flywheel and the back-up means. When the working faces of the blade-like driver are engaged by the flywheels, or one flywheel and a back-up element, the downward force (F.sub.D) applied to the driver can be stated as follows: EQU F.sub.D =XN (.mu.cos.sup.2 .theta.-SIN.theta.COS.theta.)
Where N is the squeezing force applied to the working faces of the driver, .mu. is the coefficient of friction, X is the number of flywheels (1 or 2), and .theta. is the angle of the lead-in taper or ramp on the driver.
It will be apparent from the above equation that, for a perpendicular force N, as the value of .mu. decreases, the value of N must increase in order to maintain the same downward force (F.sub.D). It will further be understood that the squeezing force N can only be applied within reasonable limits before distortion of the driver and other problems result. As a consequence, when the loss of friction is sufficient that the driving force is reduced below an acceptable limit, it is generally necessary to disassemble the tool and clean the driver and the flywheels, or the flywheel and the back-up means. This, in turn, results in downtime of the tool. The build-up of foreign material can also result in increased wear of the working faces of the driver and the working surfaces of the flywheels or the flywheel and back-up means.
U.S. Pat. No. 4,519,535 specifically addresses this problem. For this reason, its teachings are herein incorporated by reference. Briefly, this reference teaches that if the flywheel is provided with circumferential grooves (or both flywheels are provided with circumferential grooves, when two flywheels are used), a build-up of foreign materials resulting in loss of friction between the one or more flywheels and the driver will be minimized. The grooves are parallel to the parallel edges of the working surface of the flywheel. The grooves are provided in such a way that the optimum total contact area between the one or more flywheels and the driver is maintained. The grooves constitute voids along the driver-flywheel contact line into which the foreign material tends to flow. As a consequence, a positive frictional engagement of the driver by the one or more flywheels is achieved cycle-after-cycle, for a greatly extended period of time. Furthermore, the working life of the one or more flywheels, and particularly the working life of the driver, are greatly increased, due to minimization of wear of these elements.
The present invention constitutes an improvement upon the teachings of the above-mentioned U.S. Pat. No. 4,519,535. While excellent results are achieved following the teachings of this patent, it has been found that even better results can be achieved if the one or more grooves formed in the peripheral working surface of the flywheel (or the working surfaces of the flywheels) extends not only in a circumferential direction, but also at the same time in a transverse direction across the peripheral surface of the flywheel (or flywheels). In other words, the grooves taught in accordance with the present invention are angularly related to the parallel edges of the flywheel working surface in which they are formed, as will be apparent hereinafter.
It has been found that the helical or substantially helical grooves of the present invention provide a squeeze and wipe action which tends to keep the working faces of the driver and the working surface of each of the one or two flywheels cleaner for a longer period of time.
After a very large number of cycles, it has been found that the circumferential grooves, parallel to the edges of the peripheral working surface of the flywheel (as taught in U.S. Pat. No. 4,519,535), tend to wear the adjacent working face of the driver in such a way that longitudinal ridges are produced on the driver at the positions of the flywheel grooves. These ridges represent areas of the driver working face which have not worn at least as much as the remainder of the working face. Therefore, these ridges represent areas of the driver working face which have not contributed to the overall longevity of the driver. In fact, they constitute areas from which no benefit is derived. The grooves of the present invention, on the other hand, create a more uniform wear of the adjacent driver working face, thus producing longer driver work life.